I looked through the q&a and saw a couple similar situations to mine but not quite enough for me to know if I can do what I want to do without implications.  Water heater is in my mechanical room in basement.  The mech room isn’t heated, but the air handler is in there and the door to the rest of the basement isn’t an exterior door with seals etc.  The hwh is vented for intake and exhaust even though the room is exactly the minimum suggested size for not venting. So during the summer, the hybrid was using pittance for energy as the incoming air was warm and the incoming water was probably a bit warmer too.  I was outside next to the exhaust and noticed the nice cold air.  So I got to thinking why couldn’t I put in a T with a damper and run the exhaust during summer into the plenum for the house?  My concerns are whether this would mess with the hrv balancing a ton or not?  I used a/c only a handful of times during the summer as we’re more of the open window all night/early morning type.  On those days, I turn the hrv to low or bypass anyway as we have plenty of fresh air incoming. Now that it’s winter I’m wondering if I could do the opposite and put in a T for the intake and use the ambient air in the mech room to make the hwh more efficient.  I’d be sucking out some conditioned air out of the basement so it makes me wonder if doing so would negate savings on the hwh by having to have the heating system run more often, although I wouldn’t have the cold exhaust cooling the mech room either.  I’m in zone 4c with decently tight icf house.  I also wonder if I’m crazy since the hwh isn’t really that expensive to run and that I’m really just chasing cents to cover my ocd.